The present invention relates generally to adaptive equalizers and more particularly to an adaptive equalizer and method of operation that regenerates ideal real and imaginary component values from a complex valued digital signal for producing a complex valued error signal.
Data-bearing signals are often distorted by the transmission path, or channel, which they traverse between the transmitter and receiver. The distortion may be caused by the physical channel medium, such as free space, cable and the like, or nonlinear behavior of the transmitter and receiver. When such distortion consists of frequency-dependent magnitude and phase deviations of the signal, it can be corrected at the receiver by filtering the received signal with a linear filter which has inverse magnitude and phase response with respect to that of the channel. The inverse filter is called an equalizing filter, and the process of determining the filter coefficients and performing the filtering is called equalization.
The Stochastic Gradient (SG) algorithm is an efficient method to derive the value of the equalizing filter coefficients. The SG algorithm requires that a reference waveform of ideal (undistorted) data symbols be available at the receiver. When the transmitted signal contains random data unknown to the receiver, the ideal waveform can be estimated by regenerating it from the received and possibly distorted symbols. This operation can be implemented by "slicing" the received symbols to the closest ideal symbol points or levels. As long as relatively few error are made in the slicing process, the SG algorithm will converge toward the correct equalization filter.
FIG. 1 show a representative adaptive equalizer 10 for digital signals that is known in the art and generally includes a multi-tap digital filter 12 through which an unequalized digital signal passes. A slicer 14 continuously processes the equalized digital signal from the digital filter and generates ideal amplitude values of the digital levels in the signal. The ideal amplitude values are processed with the corresponding digital levels of the equalized digital signal to generate an error signal. A summation circuit 16 providing a difference output is an example of a circuit for producing the error signal. A correlator 18 receives the error signal and the unequalized digital signal. The correlator 18 performs a series of multiplications of the error signal with the unequalized digital signal, to produce a scaled output. The scaled correlator 18 output is combined with the old tap values 22 of the digital filter 12 in adder 20 to develop new tap values for the filter 12. For linear impairments in the digital signal, the circuit nullifies the effects of such impairments in the output signal.
Adaptive equalizers are employed in various configurations depending to the type of digital transmission system. In a quadrature amplitude modulation (QAM) system, for example, both the I (real) and Q (imaginary) channels are independent and data bearing. Slicers are provided for both channels for generating the ideal amplitude values for the real and imaginary data. In the advanced television (ATV) system, implemented using an 8-VSB digital transmission system, complex digital symbols (I+jQ) are generated with the in-phase or data bearing component of the signal contained in the real part of the complex signal. The imaginary part of the complex signal is used for suppressing a portion of the signal spectrum for more efficient bandwidth use. Ideal "I" or real symbols can be obtained by slicing the received "I" symbols to ideal reference levels. However, in 8-VSB, in contrast to signals such as QAM, the Q-channel signal is a function of the I-channel data symbols, and has a nearly continuous range of levels at symbol instances. Therefore, it is not possible to simply slice the received Q levels to ideal levels.
What is needed is an adaptive equalizer for equalizing complex digital signal samples where the Q-channel or imaginary component of the complex signal is a function of the I-channel or real component of the complex signal. The adaptive equalizer needs to regenerate ideal imaginary component levels from the sliced ideal real component levels of the complex signal to generate a complex error signal for driving the convergence of the filter coefficients of the equalizing filter. The adaptive equalizer needs to use the distortion information available in the Q-channel signal for improving the convergence of the equalizer process and the approximation of the inverse channel equalizing filter.